Methods and apparatus which reduce undesired noise on a control system output in a closed loop system have typically utilized some type of a noise reduction means, such as an inline filter, within the control loop itself. One disadvantage of inline filtering is that some portion of a desired control signal is typically also reduced by the action of the filter. Another disadvantage of this technique is that the filtering characteristics of the filter are normally fixed and, therefore, the filter does not respond to changes in the system which change the noise rejection characteristics of the system. Similarly, the filter does not respond to changes in the characteristics of the noise itself, resulting in more of the noise being passed through to the output of the control system.
A still further disadvantage of the systems of the prior art is that if it is desired to change the characteristics of the noise reduction means such a change may not be easily or readily accomplished inasmuch as the noise reduction means is physically placed within the control loop itself. Thus, retrofit of existing systems with a noise reduction means is either impossible or extremely difficult to achieve without inducing adverse effects on system stability and bandwidth.
A noise reduction means which has been known in the prior art is an Adaptive Noise Cancellation (ANC) system which has been employed solely in open loop types of systems. One application of ANC in the prior art has been the elimination of noise in data on transmission lines. In general, ANC involves generating an estimate of the noise and thereafter subtracting this estimate from the noise and signal on the transmission line. Thus, the degradation of the signal is substantially avoided since only the noise component is being subtracted, as opposed to inline filtering wherein both the signal and the noise are acted upon by the filter.
As can be appreciated, such a technique is more readily accomplished in an open-loop environment wherein there is, by definition, no feedback of signal and noise through the loop.
As can be further appreciated, there are a great number and variety of closed-loop control systems which would benefit from such an adaptive noise cancellation system. Examples of such systems are cross coupled loops such as inertially stabilized pointing systems, optical systems wherein the motion of a moveable optical element such as a rotating beamsplitter or a steering mirror must be precisely controlled and, also, vibration suppression systems such as those employed in large structures such as orbiting structures.
It is therefore an object of the present invention to provide for reducing undesired noise in a closed loop control system by means of an adaptive noise cancellation system.
It is also an object of the present invention to provide an adaptive noise cancellation system which measures both internal and external disturbances to the system in order to provide a disturbance cancellation output.
It is a still further object of the present invention to provide an adaptive noise cancellation system which is well suited for retrofitting an existing system or for incorporation into a new system.